Define `S_I{R,W,X}{USR,GRP,OTH}` and `S_IS{VTX,UID,GID}` for wasi.
Define `S_I{R,W,X}{USR,GRP,OTH}` and `S_IS{VTX,UID,GID}` for wasi.

WASI doesn't have full permissions setting, but it's gaining some
support, so define the POSIX names that wasi-libc defines, and let
WASI libc and WASI sort out what to do with them.

rust-lang/libc,rust-lang/libc,rust-lang/libc